Abstract: Plant cells recognize microbial patterns with the plasma‐membrane‐localized pattern‐recognition receptors consisting mainly of receptor kinases (RKs) and receptor‐like proteins (RLPs). RKs, such as bacterial flagellin receptor FLS2, and their downstream signaling components have been studied extensively. However, newly discovered regulatory components of RLP‐mediated immune signaling, such as the nlp20 receptor RLP23, await identification. Unlike RKs, RLPs lack a cytoplasmic kinase domain, instead recruiting the receptor‐like kinases (RLKs) BAK1 and SOBIR1. SOBIR1 specifically works as an adapter for RLP‐mediated immunity. To identify new regulators of RLP‐ mediated signaling, we looked for SOBIR1‐binding proteins (SBPs) in Arabidopsis thaliana using protein immunoprecipitation and mass spectrometry, identifying two G‐type lectin RLKs, SBP1 and SBP2, that physically interacted with SOBIR1. SBP1 and SBP2 showed high sequence similarity, were tandemly repeated on chromosome 4, and also interacted with both RLP23 and BAK1. sbp1 sbp2 double mutants obtained via CRISPR‐Cas9 gene editing showed severely impaired nlp20‐ induced reactive oxygen species burst, mitogen‐ activated protein kinase (MAPK) activation, and defense gene expression, but normal flg22‐induced immune responses. We showed that SBP1 regulated nlp20‐induced immunity in a kinase activity‐ independent manner. Furthermore, the nlp20‐ induced the RLP23–BAK1 interaction, although not the flg22‐induced FLS2–BAK1 interaction, was significantly reduced in sbp1 sbp2. This study identified SBPs as new regulatory components in RLP23 receptor complex that may specifically modulate RLP23‐mediated immunity by positively regulating the interaction between the RLP23 receptor and the BAK1 co‐receptor.

Key words: immune responses, pattern‐recognition receptors, receptor‐like proteins, SBPs, SOBIR1